HELP~~~How to reduce the VCO's Kv variation?

[gggould]

vco varactor bjt

Hi all,

Right now I am designing a 2GHz VCO with junction varactor.

But I found that the tuning curve's Kv variation is too hugh and the curve looks like parabolic.

Right now the Kv is about 60MHz/v at low end of the curve, but about 10MHz/v at high end.

I guess the problem comes from the varactors.

could anyone help me out?

Thanks very much,

 

[dcc]

vco without varactor

You have to check the size of the varactors. If you want to reduce the value of Kv, you have to use a smaller varactor than that you are using. Furthermore, you also have to check the bias condition of the junction varactors; pay attentation to that if the junction is reverse biased and then forward biased. The tuning curve of your VCO is determined by the value of the capacitance which is associated with the bias conditon.

 

[xtasa]

reduce kvco vco circuit

gggould,

It is normal behave for VCO as you measured.Problem is mainlly determined by varactor diode exponent characteristic. It is not so easy to obtain VCO with constant Kvco. Some ideas for solution your problem:
1. Find varactor with better characteristic, better linearity
2. Made serial connection two diodes. Take care about DC current for varactor. Frequency will going higher!!
3. Put serial capacitor in connection with varactor. At that way you can reduce Kvco change. Frequency going higher and realtive change fmax/fmin is smaller. take care about DC current current for varactor.
3. Combination 2 and 3
If you still not satisfy with results it is possible to design VCO with constant Kvco using BJT. VCO is without varactor, it use capacitance junction BCin BJT. Kvco is very constant and linearity is excellent. Problem with this type of VCO is that is not driven with voltage but current.
GL XTASA :roll:

 

[DJ]

vco kv?

1. Varactor location. The basic VCO circuit has inductor at the Base and a capacitor at the E. Varactor at the E has large influence on freq. becouse it needs capacitor at the E, and tuning this capacitor changes the freq. Large Kv. Loading varactor at the B has smaller influence becouse tuning capacitor actually change the indutance so the influence is not directly.
2. Combination of a parallel capacitor to the varactor and a series also. The series will reduce the low voltage (freq) Kv and min influence on high voltage. The prallel has min on low and mox on high voltage.
3. The varactor curve. You can find hyperabrupt as well as abrupt or linear. If the C(v) designed to work as square root and it loaded by an inductor it should be linear, means Kv constant. If you don't have problem with that think to use GaAs varactor with high capacitance ratio and use voltage divider at the Vt input.
4. Optimize the VCO parameters. Increas the E cap and reduce the B ind etc.